Not applicable.
1. Field of the Invention
The present invention relates to a test specimen, such as those normally used for monitoring spatial positioning or measuring systems, especially mobile coordinate measuring systems.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98.
The present invention relates to a test specimen comprising four shaped elements to be sensed or elements to be sensed, which will be referred to as xe2x80x9csensed elementsxe2x80x9d in the following, and six connection elements, said sensed elements being connected by said connection elements in such a way that each sensed element touches precisely three connection elements at the end faces thereof and that the sensed elements and the connection elements in their entirety form a tetrahedron on the corners of which the sensed elements are arranged.
Such test specimens are normally used for monitoring spatially positioning or measuring systems, especially mobile coordinate measuring systems. The respective test specimens for monitoring these mobile systems must not have a high own weight so that they can be transported to their location of use at an economically acceptable cost.
It will be advantageous when they are also easily demountable.
A test specimen of the type mentioned hereinbefore is known from DE 19720883 or DE 29708830.
The test specimen shown there comprises spherical sensed elements and rodshaped connection elements. The connection elements consist of carbon-fibre composites with unidirectional fibres extending in the longitudinal direction.
It is true that such carbon-fibre composites show a very small coefficient of linear expansion under standard measurement conditions, i.e. at a temperature of xe2x88x9220xc2x0 C. to +70xc2x0 C. and a humidity of 0% to 100%; this coefficient of linear expansion is, however, still too large and shows variations over this temperature range and this humidity range which are too strong for guaranteeing precise and reproducible measurement results.
Taking into account the above, it is the object of the present invention to improve the known test specimens in such a way that the measurements obtained with the aid of this test specimen provide results which, under standard measurement conditions, are independent of the ambient conditions.
According to the present invention, this object is achieved by a test specimen of the above-mentioned type which is characterized by the features that the material and/or the dimensions of the sensed elements and the material and/or the dimensions of the connection elements are adapted to one another in such a way that, under standard measurement conditions, the total coefficient of linear expansion from one sensed point to the next sensed point on different sensed elements is essentially zero.
Hence, the present invention provides a test specimen in which the sensed points of the individual sensed elements are positioned at a constant distance from one another in the measurement region in question; said distance can, on the one hand, be determined very precisely by a calibration, whereby absolutely precise measurement results are made possible, and, on the other hand, it is independent of variations of temperature and humidity in the measurement region.
According to a first alternative of the present invention, the sensed elements consist of a material with a positive or a negative coefficient of linear expansion under standard measurement conditions, the connection elements consist of a material with a negative or a positive coefficient of linear expansion under standard measurement conditions, and the dimensions of the sensed elements and the dimensions of the connection elements are adapted to one another in such a way that, under standard measurement conditions, the total coefficient of linear expansion from one sensed point to the next sensed point on different sensed elements is essentially zero.
According to a preferred further development of this first alternative, the material of the sensed elements comprises steel or ceramics and the material of the connection elements comprises a carbon-fibre composite.
Using these materials, it is possible to provide test specimens which have a particularly low weight and which can be transported to their location of use at an economically acceptable cost.
According to a second alternative of the present invention, a test specimen is provided in which the material of the connection elements comprises a glass-ceramic material and the material of the sensed elements comprises steel or ceramic or a glass-ceramic material, whereby the material of the connection elements and the material of the sensed elements are adapted to one another in such a way that, under standard measurement conditions, the total coefficient of linear expansion from one sensed point to the next sensed point on different sensed elements is essentially zero.
Especially the use of glass-ceramic materials, which have been known for a long time, permits the production of connection elements having a very stable length. In view of the infinitely small coefficient of linear expansion of glass-ceramic materials, a very good adaptation of the materials in question is achieved by using connection elements consisting of a glass-ceramic material and sensed elements consisting of materials which are normally used, such as steel or ceramics. This combination already provides a considerable improvement of dimensional stability in comparison with the formed elements known from the prior art.
Hence, a great flexibility of the material of the sensed elements can be achieved by using connection elements consisting of a glass-ceramic material, and it is still guaranteed that the test specimen according to the second alternative shows a considerably improved stability with respect to environmental influences, such as temperature and/or humidity.
The second alternative can be improved insofar as the sensed elements can also be produced from a material with an infinitely small coefficient of linear expansion, such as glass ceramics or metal alloys, e.g. Invar.
A further advantage of the use of a glass-ceramic material is that its sensitivity to humidity is lower than that of a carbon-fibre composite whose coefficient of linear expansion strongly depends on the humidity. An additional treatment of the connection elements, which is necessary in the case of carbon-fibre composites, especially when these composites are intended to be used under wet conditions, can therefore be dispensed with.
Suitable glass-ceramic materials are especially Zerodur or Ceran. One of the metal alloys having an infinitely small coefficient of linear expansion is Invar.
According to a third alternative of the present invention, a test specimen is provided in which the connection elements comprise a first material with a positive coefficient of linear expansion under standard measurement conditions and a second material with a negative coefficient of linear expansion under standard measurement conditions, and the material of the sensed elements comprises steel or ceramic or a glass-ceramic material, the distribution of the first and second materials in the connection element and the material of the sensed elements being adapted to one another in such a way that, under standard measurement conditions, the total coefficient of linear expansion from one sensed point to the next sensed point on different sensed elements is essentially zero.
This third alternative produces in principle the same effect as the second alternative, viz. that the improvement in comparison with the prior art is primarily caused by an increase in the length stability of the connection elements. The increased length stability of the connection elements permits, in turn, a very great flexibility with regard to the selection of the material and the dimensions of the sensed elements, a considerable improvement of the stability of the test specimen with regard to environmental influences, such as temperature and/or humidity, being guaranteed. By simultaneously optimizing the material of the sensed elements, a test specimen can be provided in an advantageous manner with the aid of which extremely precise measurement results can be achieved.
According to a preferred further development, steel can be used as a first material for the connection element and a carbon-fibre composite can be used as a second material for the connection element. By means of these materials a test specimen can be provided, which has a very low weight and which can be transported to its location of use easily and economically.
According to an advantageous further development of all three alternatives, the sensed elements can have a spherical shape; the connection elements can be rodshaped according to an advantageous embodiment.
Due to this special arrangement, a comparatively coarse positioning of the connection elements already results in comparatively precise positions of the sensed elements relative to one another. A further advantage of this arrangement is to be seen in the fact that all four sensed elements are normally accessible by only one sensing pin or measuring pin or tracing pin.
In accordance with an advantageous embodiment, the centres of the spheres of the sensed elements of the above-described preferred development can be located on the extensions of the axes of the rodshaped connection elements. This arrangement supports the above-described advantages.
According to another advantageous further development, the end faces of the connection elements can have a conical shape, preferably a shape that is adapted to the radius of the spherical sensed elements. This has the effect that the positioning accuracy of the sensed elements with regard to the connection elements can be increased still further.
According to a further embodiment of the above-described alternatives, the sensed elements and the connection elements can be connected releasably, e.g. by magnetic forces. This provides a mechanism which permits a particularly simple assembly and disassembly of the test specimen and, consequently, a simplified transport of said test specimen to its location of use.
For establishing the magnetic connection, magnets can be elastically bonded to the ends of the connection elements. This is advantageous insofar as burst effects, especially when materials like ceramic or glass ceramic are used, can be avoided. In the case of this embodiment the sensed elements can consist of a magnetic material or they can be provided with magnets as well.
According to one further development according to the present invention, a large-sized test specimen having a gridlike structure can be formed, which is composed of at least two test specimens according to one of the above-described test specimens, wherein respective neighbouring sensed elements are replaced by one sensed element and respective parallel neighbouring connection elements are replaced by one connection element.
In this way, a grid is formed which is composed of a plurality of adjoining tetrahedrons without any intervals. Each grid point is here occupied by a sensed element. Each sensed element within the grid is coupled with four connection elements, whereas at the grid boundaries each sensed element is coupled with three connection elements.
The advantage of this further development is that it permits the formation of large-sized test specimens with a very high stability.